Despite being considered as advanced and efficient technique for peptide production, solid-phase peptide synthesis (SPPS) is associated with drawbacks such as the excess use of building blocks as well as the formation of a huge amount of organic solvent waste especially in industrial-scale synthesis. In accordance with modern regulatory rules in pharmaceutical and cosmetic industries, organic solvents are classified into four categories: “preferred”, “usable”, “undesirable”, and “banned”. Considering common protocols applied in SPPS, many of the recommended and used solvents are rated as “undesirable”, e.g. DCM, DMF, NMP, and THF.

Therefore, ongoing attempts aim at developing alternative approaches using solvents which are permitting to reduce the risks for environment and human health. There are numerous publications describing protocols for SPPS using gamma-valerolactone, 2-methyltetrahydrofurane, propylene carbonate, N-butylpyrrolidinone, N-formylmorpholine, anisole/dimethyl carbonate, DMSO/EtOAc, and other solvents. However, the practical utility of the suggested approaches depends on the resin nature, stage of synthetic process, possibility of side reactions, physicochemical properties of solvent, and finally, price and availability.

For 2-chlorotrityl chloride resin and 4-methylbenzhydryl bromide resin, there are literature data for using inexpensive and environmentally friendly solvents at the stages of coupling and deprotection; however, not for the attachment of the first amino acid.

Structure of 2-CTC and MBH-Br resins.

Herein, we report on the replacement of DCM, THF, or DMF during resin loading by using an EtOAc (“green solvent”)/MeCN (“usable solvent”) (1:1) mixture as cost-efficient media for the first amino acid attachment, as well as on the influence of the solvent, organic base, and amino acid structure on the achieved loading value. For experimental details, please see Org. Process Res. Dev. 2022; 26(1): 144-148.

Obtained data demonstrated high loading values for Fmoc-Gly-OH on both MBH-Br and 2-CTC resin by using EtOAc, MeCN, or an EtOAc/MeCN (1:1) mixture. These results were comparable with those obtained using DCM, while the application of 2-MeTHF and DMSO/EtOAc provided only a moderate loading efficiency.

Comparing net solvents and the solvent mixture, the swelling of PS-DVB resins increases in raw MeCN < EtOAc < EtOAc/MeCN (1:1).

Selected loading protocols were applied for the comparative investigation of the reaction kinetics in DCM and the EtOAc/MeCN mixture which revealed especially high efficiency of the Fmoc-Gly-OH attachment to the MBH-Br resin by using the solvent mixture. Thus, for the MBH-Br resin, the practical utility of the suggested protocol was further confirmed by very good loading values compared to the theoretical ones for Fmoc-Leu-OH, Fmoc-Phe-OH, Fmoc-Lys(Boc)-OH, and Fmoc-His(Trt)-OH.

We believe that the suggested EtOAc/MeCN mixture can be considered as a “greener” and cost-efficient alternative to DCM and DMF for the first amino acid attachment to 2-CTC and MBH-Br resins.

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Reference:

2-Chlorotrityl Chloride and 4-Methylbenzhydryl Bromide Resin Loading Using the Mixture of Organic Solvents: A “Greener” Alternative to Dichloromethane and N,N-Dimethylformamide; M. Leko, P. Filippova, M. Dorosh, K. Rustler, T. Bruckdorfer, S. Burov; Org. Process Res. Dev. 2022; 26(1): 144-148. https://doi.org/10.1021/acs.oprd.1c00356